1. Field of the Invention
This invention relates to a high-speed wire matrix printhead where a plurality of thin wire styli are selectively impacted in order to deliver an impact to a record medium.
An earlier application, Ser. No. 354,574, and assigned to same Assignee as the instant invention, described a wire matrix printhead which is moved across a printing medium wherein selective actuation of a number of wire styli is used to imprint character indicia on the record medium as the printhead moves across it. In this earlier described embodiment, a number of efficacious features were described which provided a high-speed capability together with a minimal amount of activation and adjustment means. These features included the positioning of the wire styli in a substantially straight line yet so configured that the impact receiving ends of the wire styli were arranged along the periphery of an ellipse and the printing ends of the wire styli (impact causing ends) were aligned in a straight line. Further, the features included styli of equal length and the ability to adjust the stroke length of each individual hammer of each activated electromagnet for each stylus.
The present invention adds to this combination a unique adjustment means whereby the printout or output ends of the wire styli may be temporarily exposed for corrective grinding purposes in order to renew the working life of the wire styli.
Generally, print wires of matrix printheads are susceptible to wire wear after long periods of printing. The wire tips (output ends) shorten and begin to recess into the wire guide causing improper print impression. The wear on various styli tips is uneven since certain styli are used much more than others during the course of printing 40-50 million characters. The result is that certain tips are shorter due to wear while others are longer. It is important to restore the original wire stroke length measured from the output side of the wire guide toward the recording medium, so that character printout will be uniform and of proper print density.
Customarily, in order to accomplish the readjustment for wire tip wear, the prior art often used a method of taking the electromagnetic structures (at the input or impact receiving end of the wire styli) and readjusting them downward, in order to bring the tips or output ends of the wire styli flush with the output side of the wire guide which held the styli. This was to insure that when one individual wire stylus was activated, it would elongate itself from the wire guide face to the proper extension length designed for impact on a record medium.
Other prior art methods, when faced with a problem of the wearing and shortening of the tips of the wire styli at the output or printout end used a system of moving back the wire guide housing by such methods as removing shims or spacers which determined the position of the printing end of the wire guide housing.
In the prior art situation where each individual magnet and wire was readjusted downwardly and individually to assure "flushness" of all the print wires at the edge of the wire guide, it was a painstaking and delicate task to try to readjust each of the electromagnet actuating devices to assure flushness of each of the various print wires at the output or printout tips. In so doing, this readjustment operation, involving the lowering of the magnetic or electromagnetic actuators, would alter the design characteristic of the print wire stylus return spring which would alter the spring rate applied to the wire stylus and thus effect the time and the force with which the print wire stylus would arrive at the paper recording medium.
Since, during the life of the product, there are several readjustment periods required, then the total value of the change of the spring rate could become deleteriously significant when using the old prior art methods of readjustment.